The present invention relates generally to systems for generating microwave excited plasma discharges, and more particularly to novel materials and systems for effectively cooling high power microwave plasma tubes.
Microwave excited electrodeless discharges exhibit many attractive features for plasma excitation (continuous wave (cw) and pulsed) of low and high pressure gas in both lasers and lamps. First, such discharges appear to be inherently more stable in larger volumes and higher pressures than other types of d.c. self-sustained discharges, which stability can enable significant increases in volumetric power loading levels into the plasma. Second, the absence of metal electrodes allows discharges to be contained within either quartz or ceramic tubes or other low microwave absorbing dielectrics, and are therefore to be particularly attractive for corrosive gases such as halogens and metal vapors. Electrodeless discharges may also provide greatly enhanced stable (quiescent) plasmas in large volumes, discharge pressure scaling, increased microwave power loading per unit volume, greatly reduced gas contamination, longer lifetimes for reliable operation, and elimination of cataphoresis (particularly relevant to metal vapor lasers).
Of the aforementioned microwave discharge properties, the increase in power loading into the plasma is a prominent consideration. Increased power loadings, however, may result in temperatures (&gt;1000.degree. C. for quartz) sufficient to melt the plasma container walls (typically quartz or ceramic) or otherwise to cause structural failure (thermally induced cracks or softening) in the plasma containment apparatus. Such failures may occur for uncooled cw microwave power loadings greater than a few tens of watts/cc. Further, very high plasma tube wall temperatures can affect the kinetics of the plasma, a notable example being the CO.sub.2 laser. Consequently, gaseous or liquid cooling is essential for the plasma containment walls. Concentric high gaseous flow cooling is usually ineffective in removing excess heat because of low heat transfer between the containment walls and the gaseous coolant, and may also produce high noise levels.
Liquids have much greater cooling capacities than gases and make direct substantial contact with the plasma tube walls. Conventionally used liquids, however, do not exhibit all the desirable optical, microwave and physical properties, and are generally either high microwave absorbers (e.g., water at 2450 MHz), dangerously unsafe (e.g., CS.sub.2, CCl.sub.4), flammable (e.g., benzene, other medium weight hydrocarbons, pentane, and butane), and/or non-transmissive in the ultraviolet (UV).
Desirable properties of liquid coolant for microwave excited lamps include good transmission in the desirable spectral region (UV, visible or infrared (IR)), low microwave absorption at the microwave operating frequency, ability to withstand high cw and pulsed UV and visible radiation fluences, non-toxicity and nonflammability, large IR absorption, and desirable physical and chemical properties (low viscosity, reasonable density, low vapor pressure, large heat capacity, high thermal conductivity). The invention herein substantially solves the problems suggested above with conventional liquid cooling for microwave excited plasmas by providing coolant comprising commercially available hydraulic fluid exhibiting most of the desired optical/microwave Properties mentioned above, and can be used over a wide temperature range, -50.degree. to 150.degree. C.
It is therefore a principal object of the invention to provide safe and reliable liquid cooling for high power microwave excited plasma tubes of pulsed or cw operational mode.
It is a further object of the invention to provide liquid coolant for high power microwave excited plasma tubes with application over a wide operating temperature range.
It is another object of the invention to provide high power microwave excited plasma tube liquid coolant having low microwave absorption.
It is another object of the invention to provide liquid coolant producing significant absorption of IR radiation emitted from high power microwave excited plasma tubes.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.